Electrochromic polymers (ECPs) display a broad array of colors that can switch from the colored states to highly transmissive states. When appropriately substituted, these ECPs allow processing via spin coating, spray coating, blade coating, slot-die coating, and inkjet printing. These polymers may attain highly transmissive states with high electrochromic contrast that can be rapidly (sub-second) switched over a large number of cycles (>10000); which allow ECPs to be incorporated into display and window type electrochromic devices (ECDs) that are commercially viable.
Structural features that have been used to generate desirable visible absorption and color of the neutral state in ECPs include polymers that exploit donor-acceptor (D-A) effects, vinylene and azomethine linkers, interchange of atoms of the constituent heterocycles or that reside on β positions, variations of the repeating unit structure by the comonomer feed ratios, modification of pendant groups, and incorporation of metal-ligand interactions of metallo-supramolecular polymers. The D-A method has proven especially useful, as the donor moiety positions the HOMO while the acceptor positions the LUMO, retains the low oxidation potentials required for long term redox switching.
The transmissive oxidized states of D-A ECPs show tailing of charge carrier absorption in the near infrared into the visible spectrum, resulting in residual visible absorption that induces a blue tint to a film of the polymer. This tailing effect is evident in poly(3,4-ethylenedioxythiophene) (PEDOT), but is minimal in poly(3,4-propylenedioxythiophene) (ProDOT) ECPs that have long and bulky substituents. As exemplified by the use of an acyclic dioxythiophene (AcDOT) to develop orange and red-colored ECPs, steric control alone is capable of tuning color. It is therefore desirable to achieve ECPs of a desired color that retain the outstanding electrochromic switching to highly transmissive states that is readily achieved due to the highly electron rich nature of alkylene dioxythiophenes.